1. Field of the Invention
The present invention relates generally to a clutch assembly including a housing enclosing a hydraulically actuated clutch, and more particularly to such a clutch assembly in which pressurized air from an external source is used to regulate the level of oil within the clutch housing by returning excess oil from the housing to a fluid reservoir, such as a vehicle transmission or crankcase.
2. Description of the Related Art
In modern agricultural tractors which include selective front wheel drive, there is generally provided a hydraulic clutch in line with the main drive shaft. When the clutch is disengaged, the front wheels are freely rotating, but with the clutch engaged, a front wheel drive shaft is driven by the main drive shaft via the clutch. Such clutches are generally a part of a clutch assembly including a housing into which the main drive shaft and the front wheel drive shaft extend. The front wheel drive shaft then enters a separate differential housing from which two separate front wheel axles extend.
In such a clutch assembly, it is essential that the oil level within the clutch housing be maintained at or near a predetermined level. Too little oil can, of course, result in damage to the moving parts due to lack of lubrication. On the other hand, too much oil can decrease the efficiency of the clutch assembly, cause cavitation of the oil, "cook" the oil, and unduly heat the clutch assembly due to the large quantity of hot oil it contains. Furthermore, should the housing fill with oil, upon subsequent start-up, the internal seals can blow out due to a sudden increase in pressure, necessitating that the entire assembly be pulled apart for repairs.
However, maintaining the proper amount of oil in such a clutch housing is difficult due to the fact that hydraulically driven clutches tend to leak fluid about their seals, which fluid necessarily accumulates within the clutch housing, thus raising the oil level therein.
One known method of maintaining the proper level of oil in a differential axle housing has been to provide elastomeric, zero leak sealing rings at the interface between the differential clutch and the differential output shafts. This approach is reflected in U.S. Pat. No. 3,913,414 to Freiburger, but this solution has proven to be very expensive and imperfect since the elastomeric seals themselves tend to wear over time and thereby develop leaks.
Another known system for maintaining the proper amount of oil in a differential axle housing is that taught by U.S. Pat. No. 4,529,061 to Glasrud et al., hereinafter the '061 patent. The '061 patent uses an outside air pressure source, such as a turbocharger, to pressurize the axle housing. The amount of air needed to pressurize the housing is but a small portion of the total air output by the turbocharger, and thus engine efficiency is not unduly compromised. An oil return line between the axle housing and the transmission case connects to a stand pipe or blow back tube which extends down into the axle housing. The blow back tube ends in an open port which ports are arrayed just above a predetermined preferred static fluid level for the housing.
When the level of oil in the '061 axle housing exceeds the level of the open ports on both standpipes, then a pressure differential between the pressurized axle housing and the oil return line causes excess oil to be pumped out of the axle housing and back into the transmission case. The oil will be pumped out of the axle housing until the open port on the blow back tube is exposed.
The system disclosed in the '061 patent is generally satisfactory for detecting excess oil in static conditions. However, in the case of a front wheel drive clutch assembly, when the main vehicle transmission is engaged, the main drive shaft and the hydraulic clutch rotate together at relatively high speeds. Thus, the entire volume of oil within the clutch housing tend to be thrown against the basically cylindrical interior walls of the housing by the rotating axle, thus assuming a cylindrical shape. Even when the front wheel drive is not engaged, but the vehicle of tractor is moving, the main drive shaft and the front wheel drive clutch are rotating inside the clutch housing. Thus, truly static clutch conditions occur only when the tractor is being used for a stationary purpose, e.g. for pumping, digging, drilling, etc.
A blow back tube designed to detect static oil levels, as taught by the '061 patent, is thus not capable of detecting excess oil when the clutch assembly is in a dynamic state. Unfortunately, it is during the dynamic condition, i.e. when the main drive shaft and attached clutch is turning, and particularly when the vehicle is traversing uneven terrain, that the clutch will tend to leak oil into the clutch housing.
It is clear then that a need exists for a system which is capable of reliably detecting and evacuating excess oil in a clutch housing when the clutch is in a static and a dynamic state. The system should also be capable of distinguishing elevated oil levels resulting from operation of a vehicle on uneven terrain from truly excessive oil levels.